1. Field of the Invention
The present invention relates to a polypropylene highly spread plexifilamentary fiber, a dope used for manufacturing the fiber, and a method of manufacturing the fiber. More particularly, the present invention relates to a polypropylene plexifilamentary fiber highly spread to a three-dimensional state and having a high thermal dimensional stability, a dope including a solvent having a weak ozone layer depletion potential and used for manufacturing the fiber, and a method of manufacturing the fiber.
2. Description of the Related Art
A fiber manufactured by a flash spinning technique is known as a fiber fibrillated in a three-dimensional plexifilamentary state. The flash spinning technique is a spinning method in which a uniform solution of a polymer having a fiber-forming ability and a solvent is instantaneously extruded through a spinneret having one or more holes, at a temperature higher than a boiling temperature of the solvent and under a pressure higher than a vapor pressure of the solvent to an area under a lower pressure. The features of the fiber are disclosed in U.S. Pat. No. 3,081,519 and Japanese Examined Patent Application (Kokoku) No. 40-28125.
Namely, the fiber disclosed in U.S. Pat. No. 3,081,519 is a fiber of an organic synthetic crystalline polymer having a surface area of 2 m.sup.2 /g or more and a structure in which fibrils are spread in a three-dimensional plexifilamentary state. The fibril has an average thickness of 4 .mu. or less and an orientated structure, and is characterized in that an average orientation angle measured by an electron diffraction method is 90.degree. or less. Further this fiber is characterized in that an average orientation angle measured by an X-ray diffraction method is smaller than 55.degree., and a number of free fibrils is 50/1000 d/0.1 mm or 25/1000 d/0.1 mm, or the like. This three-dimensional plexifilamentary fiber has a non-circular cross section, and a large specific surface area, an excellent light scattering property, a superior bulkiness, and a high strength. Therefore, it is possible to make a nonwoven fabric having a high covering property and a high strength by utilizing the shape and characteristics of this fiber.
After much research, the inventors of the present application have succeeded in the development of a polypropylene three-dimensional plexifilamentary fiber having novel characteristics. The features of this polypropylene plexifilamentary fiber are that this fiber has a microwave birefringence of 0.07 or more, a superior dimensional stability in a heated environment, and a high tensile strength, a high fiber spreadability or the like. In particular, between 0.1 wt % and 10 wt % of a spreading agent is added to this polypropylene plexifilamentary fiber to apply a high fiber spreadability to the fiber, and a nucleating agent, a lubricant or a crystalline resin except a base resin, can be used in this fiber as the spreading agent. This fiber is disclosed in Japanese Unexamined Patent Publications (Kokai) No. 1-104814 and No. 1-132819, and the corresponding PCT application filed as PCT/JP 87-00808.
Known methods of manufacturing a polypropylene three-dimensional plexifilamentary fiber will be described hereafter.
These methods have been disclosed in U.S. Pat. No. 3,467,744, U.S. Pat. No. 3,564,088, U.S. Pat. No. 3,756,441 corresponding to Japanese Unexamined Patent Publication (Kokai) No. 49-42917, and Japanese Unexamined Patent Publication (Kokai) No. 62-33816 filed by the same applicant as that of the present application.
In the above known publications, a dope having an isotactic polypropylene content of between 2 wt % and 20 wt % is prepared by using a solvent, such as a 1,1,2-trichloro-1,2,2-trifluoroethane, a trichloro fluoromethane or the like, a uniform dope is made from the above dope under a pressure of a two-liquid-phase boundary pressure or more, and the uniform dope is extruded through a pressure let-down zone having a pressure of a two-liquid-phase boundary pressure or less, into an environment of an atmospheric pressure to thereby obtain a fiber. In these processes, the type of solvent, concentration of the isotactic polypropylene, MFR of the isotactic polypropylene, a temperature and a pressure of a solution prepared from the solvent and the isotactic polypropylene, a relationship between MFR, a concentration of the polypropylene and a temperature of the solution during an extruding operation, or the like have been suitably selected. In Japanese Unexamined Patent Publication (Kokai) No. 62-33816, the diameter of a nozzle is specified.
In a method of manufacturing a polypropylene three-dimensional plexifilamentary fiber disclosed in Japanese Unexamined Patent Publications (Kokai) No. 1-104814 and No. 1-132819, and the corresponding PCT application of PCT/JP87-00808, filed by the same inventors as those in the present application, a specific temperature and pressure of the solution were selected and a dope having a high viscosity was used. In particular, when manufacturing a highly spread plexifilamentary fiber, a spreading agent was added to the dope, the dope with the spreading agent was spun and then subjected to a spreading operation.
Several problems arising in the conventional polypropylene three-dimensional plexifilamentary fiber will be described hereafter.
A serious problem arising with the conventional known polypropylene three-dimensional plexifilamentary fiber is that the fiber spreadability is poor, and accordingly, it is impossible to make a nonwoven fabric having superior characteristics from the known polypropylene three-dimensional plexifilamentary fiber. With regard to the above, the polypropylene is inferior to a high-density polyethylene known to date.
The term "fiber spreadability" in the present specification means that a fiber extruded from a spinneret having a hole is separated into finer units e.g., each fibril constituting a plexifilamentary fiber.
A fiber spreading degree expressing a quality of the fiber spreadability can be evaluated by a number of free-fibrils and a fiber width thereof. The number of free-fibrils is a measure expressing a degree by which the fiber is spread to the finer unit and is shown as a number of separated fibrils per unit weight of the fiber. A larger value of the number of free-fibrils shows that the fiber is more finely separated.
The fiber width is a extent in a direction perpendicular to an axis of the fiber observed when a fiber extruded from the single hole of the spinneret is widen in a two-dimensional state in both an axial direction of the fiber and a direction perpendicular to the axial direction of the fiber. Since the fiber width depends on a quantity of the fiber used for measuring the fiber width, the fiber width is expressed as a value per unit quantity of the fiber, e.g., 10 mm/100 d. When the fiber is uniformly spread in a widthwise direction of the fiber, it is possible to approximately evaluate the fiber spreading degree only from the fiber width.
It is usually necessary for the fiber width to be 20 mm/100 d or more, to obtain a nonwoven fabric having a light weight per unit area and a high uniformity by piling a plurality of spread fibers, preferably 30 mm/100 d or more.
Nevertheless, even if the conventional known conventional polypropylene plexifilamentary fibers are spread by using an impingement plate, the obtained fiber width of the fiber is 10 mm/100 d at most.
Another problem of the known conventional polypropylene plexifilamentary fiber is that a strength of the fiber is lower. For example, Japanese Examined Patent Publication (Kokoku) No. 42-19520 disclosed a method of spreading a fiber stream extruded from a spinneret, by arranging an impingement plate in such a manner that the fiber stream is impinged on the impingement plate. A tensile strength of the fiber shown in an Example 9 in this publication is only 0.53 g/d, which is too low as a value of the fibers used in the nonwoven fabric.
As described herebefore, it has been difficult to obtain a plexifilamentary fiber having a high tensile strength and a large fiber width by using a polypropylene polymer, and although an improvement in which a nozzle of the spinneret is provided with a rectangular groove has been proposed, to solve the above problems, as disclosed in U.S. Pat. No. 3,467,744, U.S. Pat. No. 3,564,088 and Japanese Unexamined Patent Publication (Kokai) No. 49-42917, and a plexifilamentary fiber having a large fiber width can be obtained by this improvement, a tensile strength of the obtained fiber is still too low. Further, it is difficult to apply a dispersing and piling operation required when manufacturing a nonwoven fabric, which is a main application of a flash spun fiber.
Another problem of the conventional known polypropylene three-dimensional plexifilamentary fiber is that a thermal stability thereof is poor, that is, a dimensional stability under a heated atmosphere is poor, resulting in a large elongation and an easy deformation in a heated atmosphere.
As described herebefore, the same inventors as those of the present invention proposed the polypropylene three-dimensional plexifilamentary fiber having an improved tensile strength and thermal stability, and a superior fiber spreadability, and manufactured by adding a spreading agent, in Japanese Unexamined Patent Publications (Kokai) No. 1-104814 and No. 1-132819, and the corresponding PCT application No. PCT/JP87-00808. Nevertheless, the inventors found that a problem arose due to the use of the spreading agent, after filing the applications relating to the above fiber and a method of manufacturing the fiber. Namely, a clogging in a filter of a spinning apparatus is generated by the spreading agent which is little solved in a solvent under a high temperature and a high pressure, such as a benzoate, an inorganic powder, a polyamide resin or the like, and further, the nozzles of the spinneret are clogged, resulting in an obstruction of a staple spinning of the fiber.
Recently, problems regarding a solvent used for spinning a polypropylene three-dimensional plexifilamentary fiber has arisen. Namely, restriction of a production and consumption of a specified chlorinated hydrocarbon or a specified brominated hydrocarbon in which all of the hydrogen is substituted by a halogen, was started.
As the solvent used for manufacturing a polypropylene three-dimensional plexifilamentary fiber, U.S. Pat. No. 3,467,744 and U.S. Pat. No. 3,568,088 disclosed a 1,1,2-trichloro-1,2,2-trifluoroethane, and U.S. Pat. No. 3,568,088, U.S. Pat. No. 3,756,441, Japanese Unexamined Patent Publications (Kokai) No. 1-104814 and No. 1-111009 disclosed a trichlorofluoromethane.
When a nonwoven fabric, which is a main application of a flush spun fiber, is manufactured from the polypropylene three-dimensional plexifilamentary fiber by accumulating spread fibers to make a web, the spread fibers are usually electrostatically charged by a corona discharge, as disclosed in U.S. Pat. No. 3,456,156. In this case, when a combustible solvent is used, there is a risk of an ignition or an explosion of the solvent. Accordingly, a nonflammable solvent must be used for this purpose. The nonflammable solvent is generally selected from a chlorinated hydrocarbon, a fluorinated hydrocarbon, a chlorinated and fluorinated hydrocarbon. In practice, a trichlorofluoromethane, 1,1,2-trichloro-1,2,2-trifluoroethane, a dichloromethane, and a mixture of the above solvents or the like, are preferably used.
Further, to protect the ozone layer, the Vienna Treaty was adopted on 1985, followed by the Montreal Protocol in which the content of Vienna Treaty is concretely determined. Accordingly, a law stemming from the Vienna Treaty and Montreal Protocol was established in Japan, and a control based on the above law started from July, 1989. Namely, a production and a consumption of a specified material, having an especially large influence on the depletion of the ozone layer in various specified chlorinated or brominated hydrocarbons in which all of the hydrogen is substituted by the halogen and having a superior stability in the atmosphere and a large ozone layer depletion potential have been controlled.
The above-described trichlorofluoroethane and 1,1,2-trichloro-1,2,2-trifluoroethane were fall under this control, and the production and consumption of the specified chlorinated or brominated hydrocarbons in which all of the hydrogen is substituted by the halogen may be completely stopped by the year 2000.
From the above-described situation, the use of a chlorinated and fluorinated hydrocarbon in which all the hydrogen is substituted by a chlorine and a fluorine, having a superior stability in the atmosphere and broadly used as a preferable solvent for manufacturing the polypropylene three-dimensional plexifilamentary fiber, becomes difficult. Accordingly, a solvent having suitable characteristics for manufacturing the polypropylene three-dimensional plexifilamentary fiber and having a lower ozone layer depletion potential is now required.